1. Field of the Invention
This invention relates to numerical control devices for controlling machine tools, and more particularly to such devices having the function of automatically changing the tools utilized in the machine tools.
2. Description of the Prior Art
Machine tools or machining centers usually comprise tool magazines in which a variety of tools, twenty to forty in number, are held in store, and from which the tool to be used in the machining process is selected one after another by the tool change devices provided in the machine tools. Conventional numerical control devices for such machine tools could automatically select the tool from those held in the magazine, but paid no attention to the remaining life of the tools. Thus, the operators of such machine tools had to judge the remaining life of the tools from the state of the chips or files generated in the machining process, and when he judged there remained no life left for the old tool, had to replace it manually with a new one with accompanying adjustments. Thus, each replacement of old tools necessitated manual operation by the operator, which did not allow the conventional numerical control devices to control the associated machine tools without operator's intervention for a longer period of time.
A conventional numerical control device for machine tools of this type have comprised an input unit for inputting the tool select command, a memory for storing the tool select command as the tool number which is inputted from the input unit, an output unit for outputting the tool number stored in the memory to the machine tool as the magazine number, an arithmetic-logic unit, and a control unit for controlling the operations of the input unit and the memory and the outputting of the tool number from the arithmetic-logic unit to the output unit. The machine tool which is controlled by the numerical control device have comprised a tool change device for changing the tool to be used in the machining process, and the tool selected by the tool change device.
The tool changing operation by this numerical control device is as follows. The tool select command which, for example, is read out of a paper tape by a paper tape reader and is inputted through the input unit is stored in the memory unit as the tool number, and then the tool number is outputted to the output unit as the magazine number through the arithmetic-logic unit. The tool change device of the machine tool selects the tool according to the tool select command upon receiving the magazine number from the output unit, and the tool thus selected performs the machining steps prescribed by the machining program read into the numerical control device.
As the tool number is outputted as the magazine number in the conventional numerical control device as described above, the operator has to change the old tools which have reached the ends of their lives with new ones by visual inspection, for example, when the machine tool is operated by a machining program having the same tool command number for a longer period of time, or is operated continually and repeatedly by the same machining program. Thus, an operator is needed who will inspect the remaining lives of the tools, which results in the disadvantage that the associated machine tool cannot be controlled without operator's intervention for a longer period of time.